system and method to gasify aqueous urea into ammonia vapors using secondary flue gases

ABSTRACT

The present invention is a combustion system employing a urea-to-ammonia vapor reactor system. The urea-to-ammonia reactor housing enclosed in a bypass flow duct that receives a secondary flue gas stream at a split point from a main flue gas stream containing nitrogen oxides (NOx) emanating from a boiler. The bypass flow duct allows the secondary flue gas stream to flow past the enclosed reactor housing where injected aqueous urea in atomized or non-atomized form, is gasified to ammonia vapor. The resulting gaseous mixtures of ammonia, its by-products and the secondary flue gas stream subsequently rejoin the main stream, before the main flue gases are treated through a Selective Catalytic Reduction (SCR) reactor apparatus. A residence time of the secondary stream within the bypass flow duct, which may be increased by a recirculation loop, enables effective conversion of urea to ammonia to be used in the SCR apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. ProvisionalApplication No. 61/258,538, filed Nov. 5, 2009, which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention generally relates to the field of NO_(x) emissioncontrol, and more specifically to a method and system of gasifyingaqueous urea to form ammonia vapors utilized to activate SCR catalystsfor efficient treatment of flue gases comprising NO and othercontaminants.

BACKGROUND OF THE INVENTION

Use of fossil fuels (example, fuel oil) in gas turbines, furnaces,internal combustion engines and boilers, such as for power plants,industrial production, etc., results in the generation of flue gasescomprising undesirable nitrogen oxides (NO_(x), usually in the form of acombination of nitric oxide (NO) and nitrogen dioxide (NO₂). Undercertain operating conditions the NO level in a flue gas stream can belowered by reacting the NO_(x) with ammonia to produce harmless waterand nitrogen as products. This reaction can occur in the presence ofcertain catalysts, in a process known as selective catalytic reduction(SCR).

Ammonia for SCR is typically supplied by sufficiently heating aqueousurea to form gaseous ammonia. Use of the enthalpy of sufficiently hotbypass stream of flue gases, to convert a feed of urea into ammonia gas,is also known in the art. For example:

U.S. Pat. No. 4,978,514 titled, “Combined catalytic/non-catalyticprocess for nitrogen oxides reduction” to Hofmann et al., discloses a“process for reducing nitrogen oxides in a combustion effluent” that“involves introducing a nitrogenous treatment agent into the effluentunder conditions effective to create a treated effluent having reducednitrogen oxides concentration such that ammonia is present in thetreated effluent; and then contacting the treated effluent underconditions effective to reduce the nitrogen oxides in the effluent witha nitrogen oxides reducing catalyst.”

U.S. Pat. No. 5,139,754 titled, “Catalytic/non-catalytic combinationprocess for nitrogen oxides reduction” to Luftglass et al., describes “Aprocess for reducing nitrogen oxides in a combustion effluent” that“involves introducing a nitrogenous treatment agent into the effluentunder conditions effective to create a treated effluent having reducednitrogen oxides concentration such that ammonia is present in thetreated effluent; and then contacting the treated effluent underconditions effective to reduce the nitrogen oxides in the effluent witha nitrogen oxides reducing catalyst.”

U.S. Pat. No. 7,090,810 titled, “Selective catalytic reduction of NO_(x)enabled by side stream urea decomposition” to Sun et al., discloses “Apreferred process arrangement” that “utilizes the enthalpy of the fluegas, which can be supplemented if need be, to convert urea (30) intoammonia for SCR. Urea (30), which decomposes at temperatures above 140°C., is injected (32) into a flue gas stream split off (28) after a heatexchanger (22), such as a primary super heater or an economizer.Ideally, the side stream would gasify the urea without need for furtherheating; but, when heat is required it is far less than would be neededto heat either the entire effluent (23) or the urea (30). This sidestream, typically less than 3% of the flue gas, provides the requiredtemperature and residence time for complete decomposition of urea (30).A cyclonic separator can be used to remove particulates and completelymix the reagent and flue gas. This stream can then be directed to aninjection grid (37) ahead of SCR using a blower (36). The mixing withthe flue gas is facilitated due to an order of magnitude higher mass ofside stream compared to that injected through the AIG in a traditionalammonia-SCR process.”

U.S. Pat. No. 5,286,467 titled, “Highly efficient hybrid process fornitrogen oxides reduction” to Sun et al., describes “A process forreducing nitrogen oxides in a combustion effluent” that “involvesintroducing a nitrogenous treatment agent other than ammonia into theeffluent to create a treated effluent having reduced nitrogen oxidesconcentration such that ammonia is present in the treated effluent;introducing a source of ammonia into the effluent: and contacting thetreated effluent with a nitrogen oxides reducing catalyst.”

SUMMARY OF THE INVENTION

What is however needed is a system and method to provide sufficientresidence time to effectively allow for the transformation of urea intoammonia gas. Accordingly, the present invention is a novel system andmethod for enabling efficient selective catalytic reduction of NO_(x) byallowing requisite residence time for a secondary stream of sufficientlyhot flue gases to gasify aqueous urea, in a reactor assembly, intoammonia vapors. The ammonia and secondary flue gas mixture is then fedback into the main flue gas stream upstream of the SCR reactor system.

It is an object of the present invention to enable efficient selectivecatalytic reduction (SCR) of NO_(x) present in combustion productsgenerated by burning fossil fuels in boilers, gas turbines, internalcombustion engines, furnaces, and the like, collectively referred to ascombustion systems.

It is also an object of the present invention to not be limited to itsapplicability in hot exhaust gases resulting from combustion, but beapplicable to wherever SCR process is employed for the reduction ofNO_(x).

It is a further object of the present invention to allow requisiteresidence time for a secondary stream of sufficiently hot combustiongases to efficiently gasify fluids containing large quantities of liquidsuch as aqueous urea, aqueous ammonia and alcohol groups.

In one embodiment, the system and method of the present invention allowsrequisite residence time for a secondary stream of sufficiently hot fluegases, emanating from a boiler, to gasify aqueous urea, in a reactorassembly, into ammonia vapors. The ammonia and secondary flue gasmixture is then fed back into main flue gas stream upstream of a SCRreactor system to enable reduction of NO_(x) present in the main fluegas stream.

Accordingly, a urea-to-ammonia vapor reactor system in accordance withone embodiment of the present invention comprises a urea reactor housingenclosed in a bypass flow duct that receives a secondary flue gas streamseparated out from main flue gas stream at a split point. The main fluegas stream emanates from a boiler that burns fuel resulting in theproduction of combustion flue gases comprising nitrogen oxides (NO_(x)).Aqueous urea is injected, in atomized or non-atomized form, andoptionally with help of a carrier fluid such as compressed air, into thereactor housing enclosed within bypass flow duct. The bypass flow ductallows the secondary flue gas stream to flow past enclosed reactorhousing, wherein injected aqueous urea is gasified to ammonia vapor, andsubsequently enables the resulting gaseous mixtures of ammonia, itsby-products and the secondary flue gas stream to rejoin the main stream,before the main flue gases are exhausted to the atmosphere after havingbeen treated through a Selective Catalytic Reduction (SCR) reactorapparatus.

In another embodiment, a plurality of urea-to-ammonia vapor reactorsystems of the present invention are connected in series to form acascading staged-arrangement such that the mixture of ammonia vapors andsecondary flue gases resulting from a first stage forms an input to asecond stage and so on.

In a yet another embodiment, a plurality of urea-to-ammonia vaporreactor systems of the present invention are connected in aconfiguration such that each system receives an independent volume ofsecondary flue gas stream from respective split points while theresulting gaseous ammonia and flue gas mixture emanating from each ofthe plurality of systems is independently fed back into the main fluegas stream.

A desired rate of flow of gases through the plurality of stages of thesystem of the present invention is developed and maintained by the useof blowers, compressors, orifices, nozzles, valves, changes in pipediameters, piping bends, and/or combinations thereof. For instance, forboiler applications, the split point(s) may depend upon the type ofboiler being used and the temperature of flue gases at different points.In one embodiment, the split point(s) may be located within theconvection passes of the boiler and preferably upstream of an economizerif it is used.

In one embodiment, the present invention is a reactor for convertingaqueous urea into vapor ammonia, comprising: (a) an enclosure having agas flow inlet to receive a first gas stream, a gas flow outlet tooutput a third gas stream, and one or more enclosure walls that define afirst interior space disposed between said gas flow inlet and said gasflow outlet; (b) a reactor disposed within said enclosure, wherein saidreactor comprises: (i) a housing having one or more reactor walls thatdefine a second interior space and further defining a first outersurface exposed to said first interior space, wherein said first outersurface has a shape that creates a pressure differential zone betweensaid second interior space and said first interior space; a firstopening in said housing, wherein said first opening (also referred toherein as a “window”) is in said first outer surface, has across-sectional area that is less than about 35% of said first outersurface, and is disposed proximal to said pressure differential zone;and (ii) an aqueous urea inlet in fluid communication with said secondinterior space.

Optionally, a second gas stream is generated from at least some of saidfirst portion of said first gas stream and at least some of said secondportion of said first gas stream. The ammonia vapor exits said reactorhousing through a second window and mixes with said second gas stream togenerate said third gas stream. The ammonia vapor exits said reactorhousing through said second window and mixes with said second gas streamto generate a recirculation gas stream. The recirculation gas streamenters said second interior space through a first window. The exiting ofammonia vapor from said second interior space through said secondwindow, generation of said recirculation gas stream, and entrance ofsaid recirculation gas stream into said second interior space forms aconvection loop. The recirculation/convection loop is a method ofincreasing residence time. The reactor further comprises a protrudingmember, wherein said protruding member is positioned proximate to saidfirst window and extends from an outer surface of said reactor and intosaid first interior space. The second interior space is heated by atransfer of thermal energy from said first gas stream to said reactorhousing. The heating of the second interior space is sufficient togasify aqueous urea into ammonia vapor without requiring an input ofadditional thermal energy.

In another embodiment, the present invention is directed to a system forintroducing ammonia vapor into an exhaust gas stream containing NOxcomprising: (a) a reactor for converting urea to ammonia as describedherein; (b) an aqueous urea stream continuously injected into saidaqueous urea inlet; (c) a first stream comprising heated gascontinuously flowing into said gas flow inlet, around at least a portionof said reactor housing; (d) a second stream comprising mainly saidammonia vapor, wherein said second stream exists across said firstopening; (e) a third stream comprising a mixture of said heated gas andsaid ammonia vapor, wherein said third stream exists across said gasflow outlet; and (f) a port for introducing said third stream into afourth stream comprising said exhaust stream containing NOx, whereinsaid port is upstream of an SCR catalyst.

In another embodiment, the present invention is a method for producingammonia vapor comprising: (a) flowing a heating flue gas side streamaround at least a portion of a reactor to convectively heat said reactorto at least about 700° F.; (b) injecting aqueous urea into said heatedreactor, wherein step (b) is performed concurrently with step (a); (c)thermally decomposing said aqueous urea in said heated reactor until amajor portion of said urea is converted into ammonia vapor, wherein step(c) is performed concurrently with step (a); (d) withdrawing saidammonia vapor from said heated reactor, wherein step (d) is performedconcurrently with step (a); and (e) mixing said withdrawn ammonia vaporwith a portion of said heating gas.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated, as they become better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings, wherein:

FIG. 1 a shows an embodiment of urea-to-ammonia vapor reactor system ofthe present invention;

FIG. 1 b shows a series arrangement of plurality of urea-to-ammoniavapor reactor system of the present invention;

FIG. 1 c shows another configuration of plurality of urea-to-ammoniavapor reactor system of the present invention;

FIG. 2 a is a three dimensional view of the urea-to-ammonia vaporreactor assembly in accordance with one embodiment of the presentinvention;

FIG. 2 b is a first elevation side view of the reactor assembly inaccordance with one embodiment of the present invention;

FIG. 2 c is a second elevation side view of reactor assembly inaccordance with one embodiment of the present invention;

FIG. 2 d is a top plan view of reactor assembly in accordance with oneembodiment of the present invention;

FIG. 3 a is an exploded view of an embodiment of the urea-to-ammoniavapor reactor assembly of the present invention

FIG. 3 a′ is another embodiment of the invention without a hood andcorresponding window;

FIG. 3 b is an assembled three-dimensional view of an embodiment of theurea-to ammonia vapor reactor assembly of the present invention;

FIG. 3 b′ is another embodiment of the invention without a hood andcorresponding window;

FIG. 3 c is an elevation longitudinal-section view of an embodiment ofthe urea to-ammonia vapor reactor assembly of the present invention;

FIG. 3 c″ is another embodiment of the invention without a hood andcorresponding window;

FIG. 3 d is a top plan view of the flow duct lid of an embodiment of theurea-to ammonia vapor reactor assembly of the present invention;

FIG. 3 e is a second top plan view of the flow duct lid of an embodimentof the urea-to-ammonia vapor reactor assembly of the present invention;

FIG. 4 a shows a three-dimensional assembled view of reactor housing inaccordance with one embodiment of the present invention;

FIG. 4 a′ is another embodiment of the invention without a hood andcorresponding window;

FIG. 4 b shows an exploded view of reactor housing in accordance withone embodiment of the present invention;

FIG. 4 b′ is another embodiment of the invention without a hood andcorresponding window;

FIG. 4 c shows a side elevation sectioned view of reactor housing inaccordance with one embodiment of the present invention;

FIG. 4 c″ is another embodiment of the invention without a hood andcorresponding window;

FIG. 4 d shows an elevation view of first panel of reactor housing inaccordance with one embodiment of the present invention;

FIG. 4 d″ is another embodiment of the invention without a hood andcorresponding window;

FIG. 4 e shows elevation view of second panel of reactor housing inaccordance with one embodiment of the present invention;

FIG. 4 e″ is another embodiment of the invention without a hood andcorresponding window;

FIG. 4 f is a top plan view of the assembled reactor housing inaccordance with one embodiment of the present invention;

FIG. 4 g is a top plan view of the lower half assembled reactor housingwhen placed within flow duct in accordance with one embodiment of thepresent invention;

FIG. 5 a is a three dimensional view of the top lid in accordance withan embodiment of the present invention;

FIG. 5 b is a three dimensional view of the bottom of the top lid inaccordance with an embodiment of the present invention;

FIG. 5 c is a sectioned elevation view of the injector-lid assembly inaccordance with an embodiment of the present invention;

FIG. 5 d is a view from the underside of the injector-lid assembly inaccordance with an embodiment of the present invention;

FIG. 6 is a schematic representation of side stream and ammonia vaporgas flow as it passes within and through the reactor housing inaccordance with an embodiment of the present invention.

FIG. 7 shows the general direction of flow of side-stream flue gasthrough the enclosure according to one embodiment of the invention;

FIG. 8 is a cross-sectional view of the enclosure with reactor housingshowing the flow of side-stream flue gas and ammonia vapor on a planecontaining the gas flow inlet and gas flow outlet according to oneembodiment of the invention;

FIG. 9 a is a cross-sectional view of the enclosure with reactor housingshowing the flow of side-stream flue gas and ammonia vapor on a planecontaining the gas flow inlet according to one embodiment of theinvention;

FIG. 9 b is a cross-sectional view of the enclosure with reactor housingshowing the flow of mixture of side-stream flue gas and ammonia vapor ona plane containing the gas flow outlet according to one embodiment ofthe invention; and

FIG. 10 is a block diagram showing the order of principle functions ofan embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention may be embodied in many different forms, forthe purpose of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended. Any alterations and further modificationsin the described embodiments, and any further applications of theprinciples of the invention as described herein are contemplated aswould normally occur to one skilled in the art to which the inventionrelates.

FIG. 1 a shows a urea-to-ammonia vapor reactor system 100 in accordancewith one embodiment of the present invention. System 100 comprises aurea reactor housing (not shown) enclosed in a bypass flow duct 105that, in one embodiment, receives a secondary flue gas stream 112separated out from the main flue gas stream 115 at split point 110. Themain flue gas stream 115 emanates from systems that burn fossil fuel,hydrocarbon fuel. Such systems may comprise boilers, gas turbines,internal combustion (IC) engines, furnaces or any other system thatburns fossil, hydrocarbon fuel, or any other fuel that results in theproduction of combustion products comprising nitrogen oxides, as wouldbe evident to persons of ordinary skill in the art. Additionally, system100 of the present invention is not limited to its use in hotflue/exhaust gases resulting from combustion, but may be employedwherever Selective Catalytic Reduction (SCR) process is employed for thereduction of NO_(x). For the purposes of illustration the presentinvention is described with reference to boilers, however the systemsand methods of the present invention can be equally used with any othersystem that either burns fossil, hydrocarbon fuel or biomass fuel toproduce combustion gases or does not necessarily produce flue/exhaustgases as a result of combustion but where SCR process is used forreduction of NO_(x).

Thus, in one embodiment the main flue gas stream 115 emanates fromboiler 125 that burns fuel 130 resulting in the production ofcombustion/flue gases comprising nitrogen oxides (NO_(x)). Thecombustion/flue gases 115 are typically used to heat water in aplurality of heat exchanger tubes 127 before the flue gases 115 areexhausted to the atmosphere after having been treated through aSelective Catalytic Reduction (SCR) reactor apparatus 135 as is known topersons of ordinary skill in the art. Aqueous urea is injected, inatomized or non-atomized form, directly into the reactor housingenclosed within bypass flow duct 105, using urea injector 120. Thebypass flow duct 105 allows the secondary flue gas stream 112 to flowpast enclosed reactor housing, wherein injected aqueous urea is gasifiedto ammonia vapor, and subsequently enables the resulting gaseousmixtures of ammonia, its by-products and the secondary flue gas streamto rejoin the main stream 115.

During operation, an ‘x%’ volume (such as ranging from 1% to 5% of themain flue gases), of secondary flue gas stream 112 enters bypass flowduct 105 to interact with atomized or non-atomized urea that, afterreaching steady state conditions, has temperature near the secondary gasstream 112. The rate of flow of introduction of secondary flue gasstream 112 into flow duct 105 is influenced by factors such as the typeand size of boiler, rate of generation of steam from boiler, and thefuel-type used. Persons of ordinary skill in the art would appreciatethat as the secondary stream 112 enters bypass flow duct 105, itsspeed/rate of flow is altered. Thus, in one embodiment, the bypass flowduct 105 develops an independent and typically a different gas flowpattern as compared to the flow outside the duct. This altered gas flowpattern is advantageous in that, in the presence of sufficiently hotincoming secondary gas stream 112, it allows the requisite residencetime for atomized or non-atomized urea to become ammonia vapor and itby-products at steady state condition. The recirculation/convection loopis one way to increase residence time, which further enables effectiveconversion of urea to ammonia. The secondary flue gas stream 112 ispreferable within a temperature range of 700 to 950 degrees Fahrenheitto enable the reactor housing (enclosed in the bypass flow duct 105) tobe sufficiently heated. Also, the residence time ranges from 0.5 to 5seconds. Persons of ordinary skill in the art should note that thebenefit of requisite residence time enabled by system 100 of the presentinvention can be advantageously used for not only gasifying aqueousurea, such as in the present embodiment, but for efficiently gasifyingother fluids such as aqueous ammonia and hydroxyl-containing organiccompounds in alternate embodiments.

While FIG. 1 a shows the use of urea-to-ammonia vapor reactor system 100of the present invention in a single stage, persons of ordinary skill inthe art should appreciate that according to an aspect of the presentinvention and as shown in FIG. 1 b, a plurality of systems 100 can beconnected in series to form a cascading arrangement such that themixture of ammonia vapors and secondary flue gases resulting from afirst stage 141 forms an input to a second stage 142 and so on. Thus,alternate embodiments have system 100 connected in multiple stages. FIG.1 c shows a yet another configuration of use of a plurality of systems100 in accordance with another embodiment of the present invention. Inthis embodiment a plurality of systems 100, such as first 141 and second142 as shown in FIG. 1 c, are used such that each system 100 receives anindependent volume of secondary flue gas stream 112 from split points110 while the resulting gaseous ammonia and flue gas mixture emanatingfrom each of the first and second system 141 and 142 is fed back intothe main flue gas stream 115.

Persons of ordinary skill in the art should appreciate that a judicioususe of fans/blowers is utilized within connecting pipes to develop andmaintain a desired rate of flow of gases through the plurality of stagesof system 100. Referring back to FIG. 1 a, in one embodiment the mixtureof ammonia and flue gases resulting from flow duct 105 is fed into themain flue gas stream 115, by means of a blower (not shown), close to theSCR reactor apparatus 135. In one embodiment the mixture of ammonia andflue gases is directed to an ammonia injection grid upstream of the SCRreactor 135. In another embodiment a static mixer installed in thebypass connecting pipes downstream of the system 100 to further enableproper mixing of the ammonia vapors with the secondary flue gases. Inone embodiment air flow is provided by an air fan or shop compressor toprovide cooling for the urea injector nozzle as well as to provide sealair to prevent back flow of hot fluid onto the nozzle. Also, thelocation of split point(s) 110 is customizable and depends upon the typeof boiler being used and the temperature of flue gases at differentpoints. In one embodiment, the split point(s) 110 is located within theconvection passes of the boiler and preferably upstream of an economizerif it is used.

FIG. 2 a is a three dimensional view of the urea-to-ammonia vaporreactor assembly 200 in accordance with one embodiment of the presentinvention. FIGS. 2 b and 2 c are elevation side views, while FIG. 2 d isa top plan view of reactor assembly 200. Referring now to FIGS. 2 athrough 2 d, reactor assembly 200 comprises flow duct 205 that has aninlet port 207 to allow secondary flue gas stream 208 to enter thereactor assembly and an outlet port 209 to allow mixture of ammoniavapor and secondary flue gases 210 to leave the reactor assembly 200. Asvisible in FIGS. 2 c, a urea reactor housing 215 is enclosed within theflow duct 205. Urea is introduced within the reactor housing 215 by theuse of urea injector 220 that in one embodiment is attached to top lid225 of the flow duct 205. The injector 220 comprises a three-way valveimmediately upstream of the point of introduction of aqueous urea in thereactor housing 215. In one embodiment the urea injector 220 injectsatomized or non-atomized aqueous urea into the reactor housing 215 fromthe top thereby taking advantage of gravity. However, in alternateembodiments, the urea feed is provided from side or bottom of the flowduct 205.

In one embodiment the inlet and outlet ports 207, 209 comprise pipeextensions 211, 212 respectively that protrude outwards from therespective ports to facilitate connection of the ports to inlet andoutlet bypass pipes (not shown) when the reactor assembly 200 isconnected to receive bypass secondary flue gas stream of a boiler. Adrain pipe 230 passes through the bottom lid 235 of the flow duct 205.

The dimensions of the reactor elements depend on a variety of factorssuch as the type of boiler, boiler capacity, amount of ammonia to begenerated as reagents for catalytic reduction of boiler flue gaspollutants, rate of flow of flue gases, temperature of flue gases, rateof flow and composition of aqueous urea solution fed into the reactor,to name a few. In one embodiment, the length to outer diameter ratio ofthe flow duct 205 is of the order of 2.5 and may vary in a range from 2to 4 depending upon the factors aforementioned.

FIG. 3 a is an exploded view and FIG. 3 b is an assembledthree-dimensional view of an embodiment of the urea-to-ammonia vaporreactor assembly 300 of the present invention. FIG. 3 c is an elevationlongitudinal cross-sectional view, FIG. 3 d is a top plan view whileFIG. 3 e is a top view of the flow duct. Persons of ordinary skill inthe art should appreciate that such dimensions are no way limiting andvary at least according to the type of boiler or the demand onurea-to-ammonia capacity. The flow duct 305, in one embodiment, is apipe of circular cross-section that has a closed bottom 335, comprisinga hole 328 there through to accommodate a reactor housing drain pipe330, and an open top 345. A magnified view 365 of the hole 328 shows thedrain pipe 330 connected at its top end to the bottom 350 of ureareactor housing 315 and the drain-end 331 passing through hole 328 andinto a drain connector 332. As an option, a drain plug 334 can bescrewed into the drain connector 332 to act as a stop-valve.

The top opening 345, of flow duct 305, has a circular flange 347 tosecure top lid 325 thereto by means of a plurality of bolts and nuts348. FIGS. 3 d and 3 e, for assembly 300, show a circular pattern ofbolts and nuts 348 according to one embodiment. Magnified view 370 of aportion of the top lid 325 bolted to flange 347 using bolt and nut 348.Gasket 349 is used between the flange 347 and lid 325 for securetightening of the bolt and nut 348 and proper packing of the abuttingsurfaces. Flange 347 is fitted over the flow duct 305 around its topopening 345.

According to one embodiment, the reactor housing 315 of the presentinvention is fabricated with three side panels 340 to have a triangularcross-section. However, in alternate embodiments, the reactor housing315 is a circular cross-section pipe, a square cross-section housing, arectangular cross-section housing or any other suitable cross sectionhousing as would be advantageously evident to persons of ordinary skillin the art. The urea reactor housing 315 is introduced into the flowduct 305 such that it is fully enclosed within the flow duct 305. At thebottom the reactor housing 315 is affixed to the bottom plate 335 whileat the top the reactor housing 315 is secured by means of a sleeve 355(shown in magnified view 375) under the top lid 325, which sleeve isshaped and sized to form a port to hold reactor housing 315 when the toplid 325 is secured over the top opening 345 of the flow duct 305. Thetop lid 325 also comprises a centrally bored through-hole 360 to fixedlyhold an integrated injector assembly 320 and allow urea to be introducedinto the reactor housing 315. FIG. 3 a′ is an exploded view and FIG. 3b′ is an assembled three-dimensional view of the urea-to-ammonia vaporreactor assembly 300 according to another embodiment of the presentinvention. FIG. 3 c″ is an elevation longitudinal cross-sectional viewof this further embodiment. These figures show a variation of theembodiment shown in FIG. 3 a, FIG. 3 b, and FIG. 3 c, without hood 379and window (shown in FIGS. 4 a and 4 b as window 476).

FIG. 4 a is a three-dimensional assembled view while FIG. 4 b is anexploded view of reactor housing 415 according to an embodiment of thepresent invention. FIG. 4 c is a side elevation sectional view, FIGS. 4d and 4 e are elevation views of first and second panels, respectively,FIGS. 4 f and 4 g are top plan views of the assembled reactor housing415, itself and its lower half portion, when placed within flow duct405, respectively. The reactor housing 415 comprises three panels—afirst panel 471, a second panel 472 and a third panel 473; a bottompanel 474 with a centrally bored drain hole 493; three panel insidecorners 475 and a drain pipe 430. The first panel 471 comprises a window476 and two additional narrow windows 478 below. A hood 479 is attachedto panel 471 above the window 476 such that it covers only a part of thewindow 476 longitudinally but overhangs to cover the breadth of thewindow 476 fully. FIGS. 4 a′, 4 b′, 4 c″, 4 d″ and 4 e″ show aspects ofthe reactor housing 415 without hood 479 and window 476, in accordancewith another embodiment of the present invention.

According to an embodiment of the present invention, the three panels471, 472, and 473 are of the same height. Panels 472 and 473 have thesame width which is more than the width of the first panel 471. Also,the second and third panels 472 and 473 have bottom cuts 480 and flanges482 with holes 483 there through. Panel 471 also can have a portion ofthe bottom removed to accommodate flue gas passage as needed. The bottompanel 474 has recessed corners to accommodate the panel inside corners475 thereon. FIG. 4 f show the top plan view of the reactor housing 415when the three panels 471, 472 and 473 are attached to the three cornerbottom panel 474, using the panel inside corners 475. Magnified view 490shows how an inside corner 475 is used to connect any two panels. Thedrain pipe 430 connects the drain hole 493 of the bottom panel 474 withthe hole bored in the bottom plate 435 of the flow duct 405.

As shown in magnified view 495 of FIG. 4 g the reactor housing 415stands on top of the bottom plate 435 of the flow duct 405 and may besecured by means of fasteners in holes 483 of flanges 482 of the secondand third panels 472, 473. Referring again to FIG. 4 g, persons ofordinary skill in the art should note than in one embodiment of thepresent invention, the reactor housing 415 is enclosed within the flowduct 405 such that the corner 492 formed by the intersection of thesecond and third panels 472 and 473 points towards the inlet port 407for flue gases. As a result, the first panel 471 (comprising window 476and two narrow windows 478) faces the outlet port of the flow duct 405.

This positioning of the panels and therefore that of the reactor housing415 relative to the inlet and outlet ports of the flow duct 405 alongwith the positioning of the window 476 and hood 479 relative to theoutlet port is advantageous in that it enables the retention time forthe injected urea to be heated indirectly enough by the flue gases tobecome ammonia vapor. Thus, during operation secondary flue gases enterthe flow duct 405 through inlet port 407 to impinge on corner 492thereby getting bifurcated along the second and third panels 472 and 473to reach the oppositely positioned first panel 471. As the flue gasesturn the corners formed by panels 472 and 471, and 473 and 471, inpreparation to exiting the flow duct 405 through outlet port 309, suchflow movement induces a differential pressure environment near the twonarrow windows 478 from which vaporized urea-to-ammonia fluid inside thereactor housing is steadily drawn out and subsequently is entrained byboth flue gas streams. While the gaseous mixture of ammonia and fluegases prepare to leave the reactor flow duct 405 through the outlet port309, a small portion of said flue gases is retained by the hood 479 andis forced to seep into the reactor housing 415 through window 476.

The above described flow arrangement of flue gases also allows forproper direct heating of all the three panels of the reactor housing 415as well as to induce a circulating convection loop between the reactorhousing 415 and the flow duct 405 to provide additional energy to theatomized aqueous urea droplets (or non-atomized aqueous urea) injectedfrom the top causing them to be gasified into ammonia vapor and otherby-products. The set up of this circulating convection loop, as well asby controlling the flow velocity of the flue gases through the flow duct405, further ensure excess urea-to-ammonia retention time is achievedwithin the reactor housing. In one embodiment, the only thermal energysource required to gasify aqueous urea into ammonia is the thermalenergy, or heat, contained by the input flue gas stream.

Referring to FIG. 6, the convection loop created by and within reactor615 is shown. Reactor gas 687 combines with re-circulated gas 686, tocreate a reactor output gas flow 689, which leaves through a firstreactor window 678. A first portion of the reactor output 689 gas leavesthe system as reactor discharge gas 696 while a second portion of thereactor output gas 689 is re-circulated back into the reactor 615,through a second reactor window 676, as re-circulated gas 686. It shouldbe appreciated that the terms “first” and “second”, when used to referto similar elements such as reactor housing windows, are merely intendedto indicate a difference or distinction between the two elements, not anorder, an absolute required number of elements, or a required sequence.Accordingly, the terms “first” and “second” can be used interchangeablywhen referring to the two reactor housing windows.

Referring back to FIG. 3 c, it should be noted that the hood 379 ispositioned such that it lies somewhat below the center 301 of the outletport 309 so that the mixture of ammonia vapor and flue gases emanatingfrom below do not directly flow out through outlet port 309 unhindered.It should be appreciated that this particular arrangement, thoughadvantageous, is in no way limiting and therefore alternate embodimentsmay have other positioning of the reactor housing relative to the inletand outlet ports of the flow duct without departing from the scope ofthe present invention.

FIG. 5 a is a three dimensional view of the top lid 525 comprising acentrally bored through-hole 560 to accommodate inlet pipe 506 of theurea injector assembly 520. FIG. 5 b is a three dimensional view of thebottom of the top lid 525. The bottom of the lid 525 has a circularinsulated liner 510 as shown in FIG. 5 c, the elevation cross-sectionalview of the top lid 525. The insulated liner is secured to the bottom ofthe lid by a metal plate. The top lid 525 also provides an instrumentsensing access port 507 to the reactor housing 415 allowing continuousmonitoring of the operating condition such as temperature, pressure andspecies within the reactor housing 415. FIG. 5 d is a sectioned bottomview of the injector-lid assembly 500 and the top portion of the reactorhousing 415.

When the injector-lid assembly 500 is affixed on the flow duct 505, theliner 510 fits inside the flow duct 505 while the lid 525 rests over theflange 547 with gasket 549 there between to improve packing of theabutting surfaces when the lid 525 is secured to the flange 547 by meansof a plurality of bolts and nuts. As shown in FIGS. 5 b and 5 d, sleeve555 is formed and attached to the bottom of the liner 510 around thehole 560. In one embodiment, the sleeve 555 is triangular in shape toaccommodate the top portion of reactor housing 515 that in oneembodiment has a triangular cross-section. FIG. 5 d more clearly showthe triangular cross-sectioned reactor housing 515 fitted into the portformed by the triangular sleeve 555. Persons of ordinary skill in theart would understand that the shape of the sleeve is customizableaccording to the shape of the cross section of the reactor housing 515in alternate embodiments.

Turning to FIG. 7, shown is the general flow of side-stream flue gas 701through an enclosure 702 containing the urea-to-ammonia reactor (notshown). A first gas stream 703 (e.g., a side-stream flue gas streamderived from a main flue gas stream) enters the enclosure (or chamber)702 via a gas flow inlet 703, traverses the enclosure, wherein it ismixed with ammonia vapor, and the resulting mixture flows out of theenclosure via a gas flow outlet 705 as a third gas steam 711 forexample. In preferred embodiments, at least one enclosure wall 708 ofthe enclosure defines a first interior space 710 disposed between thegas flow inlet and gas flow outlet. In certain embodiments, theenclosure wall 708 is cylindrical and circumscribes the first interiorspace, and along with a first enclosure end 706 and a second enclosureend 707 defines the first interior space. In other embodiments, the wallspherical, cubic, or the like. Preferably, the first end 706 is proximalto the gas flow inlet (relative to said gas flow outlet) and the secondend 707 is proximal to the gas flow outlet (relative to said gas flowinlet).

Turning to FIG. 8, shown is a cross-sectional view of the enclosure 801that encompasses a reactor 800 having a reactor housing 830, the viewshowing the flow of inlet flow stream 810, the side-stream flue gas 811,and ammonia vapor 812, the view being on a plane containing the gas flowinlet 804 and gas flow outlet 805 according to one embodiment of theinvention. FIGS. 9 a and 9 b show a cross-sectional view of theenclosure 801 with reactor housing 830 showing the inlet flow stream 810and the split side-stream flue gas 811 and ammonia vapor 913 on a planecontaining the gas flow inlet 804 according to one embodiment of theinvention; and a cross-sectional view of the enclosure 801 with reactorhousing 830 showing the flow of mixture of side-stream flue gas andammonia vapor 914 on a plane containing the gas flow outlet 805according to one embodiment of the invention; respectively.

Here, the enclosure 801 defines a first interior space 820. The reactorhousing 830 has one or more walls 831 that are disposed in relationshipto each other to define a second interior space 832. The reactor housingwalls 831 further define a first outer surface 903 exposed to said firstinterior space 820. The first outer surface also has one or moreopenings (e.g., windows) that serve as an interface between the firstinterior space and the second interior space and also allow the twospaces to be in fluid communication with each other. Preferably thecross sectional area of the windows is less than about 35% of the totalsurface area of the first outer surface, and more preferably less thanabout 10%. In certain embodiments, the one or more of the openings eachhave a major direction and a minor direction and have a rectangularprofile, wherein the major direction is parallel to the reactor majoraxis 843. In certain preferred embodiments, one or two of the openingsare proximal to the first reactor end relative to the second reactorend.

Preferably, at least a portion of the surface 903 has a shape, such asan angle, that creates a pressure differential zone 821 between thefirst interior space 820 and the second interior space 832. Thelocation, size, and orientation of the pressure differential zone 821can vary with reactor dimensions and operating conditions. They can alsovary even while the system is operating under steady state conditions.Preferably, the zone is formed and maintained at or near an interface ofthe first interior space with the second interior space, such as at anopening (e.g., window) 806. In certain instances, the zone is a smallpocket that forms within a gas stream. Flow separation on a givensurface is due to a change of direction which creates a pressuredifferential between the gas stream and the nearby surface. Sincepressure is a force acting upon a surface, in the case of a flowseparation, a “reverse flow field” is established and therefore, theflow direction on the separated surface region can be generallycharacterized as “lifted away” from the surface and as such, produces alowest pressure point but not necessarily negative (vacuum) within theentire enclosure. The windows are designed and positioned in specificareas to be proximal to the zone, thus the pressurized decomposed ureagases in the second interior space cross into the first interior spaceby the action of the external reversed flow stream (entrainment).Therefore, fluid from the urea reactor (second interior space) to theflow duct (first interior space) is only one direction under theseconditions.

The reactor further contains an aqueous urea inlet 803, such as ainjector, a nozzle, or the like, that is located in a position to allowaqueous urea, including atomized urea, to enter the second interiorspace.

The gas flow inlet 804 has a inlet major axis 841 and the gas flowoutlet has an outlet major axis 842. The reactor housing 830 has areactor major axis 843 that is orthogonal to said inlet major axis, andpreferably has a triangular profile 940 about said reactor major axis843. The triangular profile preferably has a leading vertex 941, a firstdrag vertex 942, and a second drag vertex 943, wherein the leadingvertex is proximal to the gas flow inlet 804 relative to the first andsecond drag vertices. Preferably, at least a portion of the first outersurface 903 is opposite to the leading vertex. As used herein, the termvertex mean an edge formed by the intersection of faces or facets of anobject. In certain preferred embodiments, the triangular profile 940 hasan isosceles shape, wherein the leading vertex has an interior angle 946that is less than 60°. Preferably, the reactor housing has a firstreactor end 851 and a second reactor end 852 wherein the first andsecond ends have centroids 904 along the reactor major axis 843 and thefirst reactor end 851 is proximal to gas flow inlet 804, relative to thegas flow outlet 805, and the second reactor end 852 is proximal to thegas flow outlet 805, relative to the gas flow inlet 804.

The reactor system of the present invention is advantageous in thatprovides a relatively more compact design compared to conventional ureato ammonia conversion systems. Accordingly, another aspect of thepresent invention is a reactor system having, in addition to certainfeatures mentioned above, (c) a main flue gas conduit; (d) an SCRcatalyst; and (e) a flue gas side-stream conduit having (i) a firstportion disposed upstream of said gas flow inlet and fluidly connectedto said gas flow inlet and a flue gas conduit at a first position, and(ii) a second portion downstream of said gas flow outlet and fluidlyconnected to said gas flow outlet and said flue gas conduit at a secondposition, wherein said second portion downstream of said first positionand upstream of said SCR catalyst, relative to a flow of flue gasthrough said main flue gas conduit; and preferably having a linear flowdistance between said gas flow inlet and said second position that isless than about 40 times, more preferably less than about 30 times, evenmore preferably less than about 25 times, and most preferably less thanabout 20 times, an average diameter of said side-stream conduit.

The present invention may be embodied in other specific forms withoutdeparting from its essential characteristics. The described embodimentis to be considered in all respects only as illustrative and not asrestrictive. The scope of the present invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of the equivalence ofthe claims are to be embraced within their scope.

1. A reactor for converting aqueous urea into vapor ammonia comprising:a. an enclosure having a gas flow inlet to receive a first gas stream, agas flow outlet to output a third gas stream, and one or more enclosurewalls that define a first interior space disposed between said gas flowinlet and said gas flow outlet; b. a reactor disposed within saidenclosure, wherein said reactor comprises: i. a housing having: one ormore reactor walls that define a second interior space and furtherdefining a first outer surface exposed to said first interior space,wherein at least a portion of said first outer surface has a shape thatcreates a pressure differential zone between said second interior spaceand said first interior space, a first opening in said housing, whereinsaid first opening is in said first outer surface, has a cross-sectionalarea that is less than about 35% of said first outer surface, and isdisposed proximal to said pressure differential zone; and ii. an aqueousurea inlet in fluid communication with said second interior space. 2.The reactor of claim 1, wherein said gas flow inlet has a inlet majoraxis and said housing has a reactor major axis orthogonal to said inletmajor axis and a triangular profile about said reactor major axis. 3.The reactor of claim 2 wherein said triangular profile has a leadingvertex, a first drag vertex, and a second drag vertex, wherein saidleading vertex is proximal to said gas flow inlet relative to said firstand second drag vertices, and wherein said first outer surface isopposite to said leading vertex.
 4. The reactor of claim 3 wherein saidtriangular profile is isosceles, said leading vertex has an interiorangle less than about 60°, and said inlet major axis bisects saidleading vertex.
 5. The reactor of claim 1 wherein said gas flow outlethas a outlet major axis that is parallel to said inlet major axis andsaid gas flow outlet is proximal to said first exterior surface relativeto other surfaces of said triangular profile.
 6. The reactor of claim 5wherein said reactor housing has a first end and a second end, whereinsaid first and second ends have a centroid along said reactor major axisand said first end is proximal to said gas flow inlet relative to saidgas flow outlet and said second end is proximal to gas flow outletrelative to said gas flow inlet.
 7. The reactor of claim 6 wherein saidfirst opening is has a first major direction and a first minordirection, and said major direction is parallel to reactor major axisand is adjacent to, but distinct from, said first drag vertex.
 8. Thereactor of claim 7 wherein said first opening is proximal to said firstend relative to said second end.
 9. The reactor of claim 8 furthercomprising a second opening in said first outer surface, wherein saidsecond opening has a cross-sectional area that is less than about 35% ofsaid first outer surface, has a second major direction and a secondminor direction, and said major direction is parallel to said reactormajor axis and is adjacent to, but distinct from, said second dragvertex.
 10. The reactor of claim 1 further comprising: c. a main fluegas conduit d. an SCR catalyst; and e. a flue gas side-stream conduithaving i. a first portion disposed upstream of said gas flow inlet andfluidly connected to said gas flow inlet and a flue gas conduit at afirst position, and ii. a second portion downstream of said gas flowoutlet and fluidly connected to said gas flow outlet and said flue gasconduit at a second position, wherein said second portion downstream ofsaid first position and upstream of said SCR catalyst, relative to aflow of flue gas through said main flue gas conduit.
 11. The reactor ofclaim 10 having a linear flow distance between said gas flow inlet andsaid second position that is less than 25 times an average diameter ofsaid side-stream conduit.
 12. The reactor of claim 1 further comprising:c. a third opening in said first outer surface, wherein said thirdopening has a cross-sectional area that is greater than saidcross-sectional areas of said first and second openings and wherein saidthird opening is separate from said first and second openings; and d. aprotruding member extending from said first outer surface and positionedin said first interior space, exterior to said second interior space,and proximal to said third opening.
 13. A method for producing ammoniavapor comprising: a. flowing a heating flue gas side stream around atleast a portion of a reactor to convectively heat said reactor to atleast about 700° F.; b. injecting aqueous urea into said heated reactor,wherein step (b) is performed concurrently with step (a); c. thermallydecomposing said aqueous urea in said heated reactor until a majorportion of said urea is converted into ammonia vapor, wherein step (c)is performed concurrently with step (a); d. withdrawing said ammoniavapor from said heated reactor, wherein step (d) is performedconcurrently with step (a); and e. mixing said withdrawn ammonia vaporwith a portion of said heating gas.
 14. The method of claim 13, whereinsteps (a), (b), (c), (d), and (e), are performed on a continuous basis.15. The method of claim 14, wherein said withdrawing is performed byforming a pressure gradient between said flowing heating gas and ammoniavapor, respectively.
 16. The method of claim 15, wherein said pressuregradient is produced by flowing said heating gas around at least aportion of said reactor.
 17. The method of claim 16, wherein at least 99weight percent of said urea is vaporized during step (c).
 18. The methodof claim 13 wherein steps (a), (b), (c), (d), and (e) are performedwithin an enclosure.
 19. A system for introducing ammonia vapor into anexhaust gas stream containing NOx comprising: a. a reactor according toclaim 1; b. an aqueous urea stream continuously injected into saidaqueous urea inlet; c. a first stream comprising heated gas continuouslyflowing into said gas flow inlet, around at least a portion of saidreactor housing; d. a second stream comprising mainly said ammoniavapor, wherein said second stream exists across said first opening; e. athird stream comprising a mixture of said heated gas and said ammoniavapor, wherein said third stream exists across said gas flow outlet; andf. a port for introducing said third stream into a fourth streamcomprising said exhaust stream containing NOx, wherein said port isupstream of an SCR catalyst.
 20. The system of claim 19 wherein saidheated gas is a side stream extracted from said exhaust gas streamupstream of said port.